Datasheet

OPA2673
www.ti.com SBOS382F –JUNE 2008–REVISED MAY 2010
BOARD LAYOUT GUIDELINES peaked frequency response. The 402Ω feedback
resistor used in the Typical Characteristics at a gain
Achieving optimum performance with a
of +4V/V on ±6V supplies is a good starting point for
high-frequency amplifier such as the OPA2673
design. Note that a 511Ω feedback resistor, rather
requires careful attention to board layout parasitic and
than a direct short, is recommended for the unity-gain
external component types. Recommendations that
follower application. A current-feedback op amp
optimize performance include:
requires a feedback resistor even in the unity-gain
follower configuration to control stability.
a) Minimize parasitic capacitance to any ac ground
for all of the signal I/O pins. Parasitic capacitance on
d) Connections to other wideband devices on the
the output and inverting input pins can cause
board may be made with short direct traces or
instability; on the noninverting input, it can react with
through onboard transmission lines. For short
the source impedance to cause unintentional band
connections, consider the trace and the input to the
limiting. To reduce unwanted capacitance, a window
next device as a lumped capacitive load. Relatively
around the signal I/O pins should be opened in all of
wide traces (50mils to 100mils, or 1,27mm to
the ground and power planes around those pins.
2,54mm) should be used, preferably with ground and
Otherwise, ground and power planes should be
power planes opened up around them. Estimate the
unbroken elsewhere on the board.
total capacitive load and set R
S
from the plot of
Differential R
S
vs Capacitive Load (Figure 27). Low
b) Minimize the distance (< 0.25in, or 6,350mm)
parasitic capacitive loads (< 5pF) may not need an
from the power-supply pins to high-frequency 0.1mF
R
S
because the OPA2673 is nominally compensated
decoupling capacitors. At the device pins, the ground
to operate with a 2pF parasitic load. If a long trace is
and power-plane layout should not be in close
required, and the 6dB signal loss intrinsic to a
proximity to the signal I/O pins. Avoid narrow power
doubly-terminated transmission line is acceptable,
and ground traces to minimize inductance between
implement a matched impedance transmission line
the pins and the decoupling capacitors. The
using microstrip or stripline techniques (consult an
power-supply connections (on pins 7 and 14 for a
ECL design handbook for microstrip and stripline
QFN package) should always be decoupled with
layout techniques). A 50Ω environment is normally
these capacitors. An optional supply decoupling
not necessary onboard. In fact, a higher impedance
capacitor across the two power supplies (for bipolar
environment improves distortion; see the distortion
operation) improves second-harmonic distortion
versus load plots. With a characteristic board trace
performance. Larger (2.2mF to 6.8mF) decoupling
impedance defined based on board material and
capacitors, effective at a lower frequency, should also
trace dimensions, a matching series resistor into the
be used on the main supply pins. These can be
trace from the output of the OPA2673 is used, as well
placed somewhat farther from the device and may be
as a terminating shunt resistor at the input of the
shared among several devices in the same area of
destination device. Remember also that the
the PCB.
terminating impedance is the parallel combination of
c) Careful selection and placement of external
the shunt resistor and the input impedance of the
components preserve the high-frequency
destination device.
performance of the OPA2673. Resistors should be
This total effective impedance should be set to match
of a very low reactance type. Surface-mount resistors
the trace impedance. The high output voltage and
work best and allow a tighter overall layout. Metal film
current capability of the OPA2673 allows multiple
and carbon composition axially-leaded resistors can
destination devices to be handled as separate
also provide good high-frequency performance.
transmission lines, each with respective series and
Again, keep the leads and PCB trace length as short
shunt terminations. If the 6dB attenuation of a
as possible. Never use wire-wound type resistors in a
doubly-terminated transmission line is unacceptable,
high-frequency application. Although the output pin
a long trace can be series-terminated at the source
and inverting input pin are the most sensitive to
end only. Treat the trace as a capacitive load in this
parasitic capacitance, always position the feedback
case, and set the series resistor value as shown in
and series output resistor, if any, as close as possible
the plot of Differential R
S
vs Capacitive Load
to the output pin. Other network components, such as
(Figure 27). However, this approach does not
noninverting input termination resistors, should also
preserve signal integrity as well as a
be placed close to the package. Where double-side
doubly-terminated line. If the input impedance of the
component mounting is allowed, place the feedback
destination device is low, there is some signal
resistor directly under the package on the other side
attenuation because of the voltage divider formed by
of the board between the output and inverting input
the series output into the terminating impedance.
pins. The frequency response is primarily determined
by the feedback resistor value as described
previously. Increasing the value reduces the
bandwidth, whereas decreasing it gives a more
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